# schrenk approximation

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#### Pops

##### Well-Known Member
HBA Supporter
Log Member
Fuselage 36" with a constant 14" cord. Short coupled.

#### ragflyer

##### Well-Known Member
your shear load at the root should be equal to your lift load and BM should be equal to about half semispan times lift load. Find and fix the issue and you should see a significantly lower BM.

#### ragflyer

##### Well-Known Member
The calculations are for half the wing but you have the full 25lbs in your spread sheet......as a quick check assuming 1.4G limit load (you should at least double it) at the root your the limit shear load should be 1.4*25/2= 17.5lbs and BM should be 0.46*35*1.4*25/2=306 inch/lbs

#### mcrae0104

##### Well-Known Member
HBA Supporter
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Well, considering we do not plan to do any extreme maneuvers in the air; only take off, make a large circle in the air, then land, per competition requirements.
As long as you can do it in dead calm air? That's an academic exercise. Maybe you could just turn in your calculations and call it good, without having to bother with building anything.

#### rich_aero

##### Member
As long as you can do it in dead calm air? That's an academic exercise. Maybe you could just turn in your calculations and call it good, without having to bother with building anything.
I have to build it. Trying to learn aerodynamics to the best of my ability as I go, was assigned a project I have no previous education on.

#### wsimpso1

##### Super Moderator
Staff member
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rich_aero - you are brave both to take on such a project.

Dr Mac Cready's famous airplanes had small FOS too. You may find that the smallest spars you can conscience combined with the rest of the wing structure will end up with substantially more g capability. I would still make my first flights inside a gymnasium if I could. If they go well and you can fly 45 degree banked turns, then you could outside on calm early mornings or afternoons just before dusk.

rich_aero and I have sorted out the details of the program he emulated. Its results look just like mine... I believe it is the one in the old PFA document on this topic. It has been posted elsewhere on hba.com, so I shall not repeat the posting here.

Billski

#### TFF

##### Well-Known Member
Technically these projects are meant to fail. A few will work, but the schooling if the project is figuring out the failure. Forensic learning.

I can’t do the calculations, but I have built somewhere between 20-30 RC planes that size and could whittle one out that would work in about a week. But that is not what you are supposed to learn about.

Is there time for testing? Do you have a budget for two? Anyone who has flown RC for any amount of time know the first flights are dialing it in. Just showing up day of competition, unless that’s what the assignment says to do, will be spectacular YouTube content for just about everyone. Don’t feel bad, as that is probably the expectation of the teacher. Unlucky testing also might be a total loss that you might have to recover from, where if it crashes in comp day you still will get a good grade. The lessons are not always what you think they are. Good paperwork and a crashed plane will be more valuable than a winning plane and bad paperwork. Then you have the giants. I believe Burt Rutan won the senior age class payload free flight championship in 1960.

#### proppastie

##### Well-Known Member
Log Member
The calculations are for half the wing but you have the full 25lbs in your spread sheet......as a quick check assuming 1.4G limit load (you should at least double it) at the root your the limit shear load should be 1.4*25/2= 17.5lbs and BM should be 0.46*35*1.4*25/2=306 inch/lbs
that is the number I get (306.25in/lb) for the area platform max moment semispan to CL .......what does the Schrenk come up with....too complicated for me but interested to see the difference.

#### rich_aero

##### Member
After some generous help from wsimpso1, I was able to create more accurate shear and moment plots for the plane considering the following specs

wingspan 5.833 ft
wing area 6.8 ft^2
constant chord length of 1.1667 ft (14 inches)
take off weight 25 lbs
Schrenk Lift 2(25)=50 lbs, considering 2 g loads. (yes, I'm aware this may be a bit low, the g force can be changed at anytime to update the plots accordingly.

see files attached.

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#### rich_aero

##### Member
Technically these projects are meant to fail. A few will work, but the schooling if the project is figuring out the failure. Forensic learning.

I can’t do the calculations, but I have built somewhere between 20-30 RC planes that size and could whittle one out that would work in about a week. But that is not what you are supposed to learn about.

Is there time for testing? Do you have a budget for two? Anyone who has flown RC for any amount of time know the first flights are dialing it in. Just showing up day of competition, unless that’s what the assignment says to do, will be spectacular YouTube content for just about everyone. Don’t feel bad, as that is probably the expectation of the teacher. Unlucky testing also might be a total loss that you might have to recover from, where if it crashes in comp day you still will get a good grade. The lessons are not always what you think they are. Good paperwork and a crashed plane will be more valuable than a winning plane and bad paperwork. Then you have the giants. I believe Burt Rutan won the senior age class payload free flight championship in 1960.
We plan to build 3 planes and do as much flight testing as possible in the given amount of time we have. You are correct that MANY teams show up to the competition and crash their plane on the first flight. We are trying not to be one of those teams...

#### proppastie

##### Well-Known Member
Log Member
aprox. 20 % conservative Uniform to Elliptical for a rectangular platform.....had me going there until I noticed you upped it to 2G...